This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a new sandwich cathode design having a first cathode structure of a first thickness sandwiched between two current collectors and with a second cathode structure of a second thickness in contact with the opposite sides of the current collectors. The active material of the first and second structures is the same. The only difference being that the first thickness is different than the second thickness, and the second thicknesses of the two second cathode structures are preferably the same. The present cathode design is useful for high discharge rate applications, such as experienced by cells powering an implantable medical device.
In conventional cathode designs, the thickness of the electrode may affect the cell capacity and cell discharge rate capability. In principle, the thicker the cathode in a defined cell volume, the higher the cell capacity. However, in some cases, the cathode discharge rate capability is decreased significantly when a relatively low conductivity cathode active material is used. The reason for this is that the resistance of any electrode is proportional to the thickness or distance from its outer surface to the current collector. Electrode thickness is particularly important and problematic in cell design modeling projections. In some situations, differing electrode thicknesses may make it very difficult to predict how a particular active material which is relatively resistive, either electrically or tonically, may perform. As a result, the active material is precluded from consideration as being impractical as a power source for some design applications.
In the double screen design of the present invention, however, the so called xe2x80x9cfirst cathode structurexe2x80x9d disposed between the opposed current collectors is of a different thickness than the so called xe2x80x9csecond cathode structurexe2x80x9d contacting the outsides of the two current collectors. Preferably, the thicknesses of the second cathode structures contacting the outside of the current collectors is the same. Then, the first cathode structure sandwiched between and in direct contact with the cathode current collectors can be either thicker or thinner than the thickness of the second structures, depending on a particular application. This makes it easier to model the discharge rate capability of the cell, even for a new design or application.
One type of chemistry in which the present configuration is particularly useful is a lithium/silver vanadium oxide (Li/SVO) cell. For this couple, silver vanadium oxide cathode active material, possibly devoid of a binder and a conductive diluent, in the form of a first structure of a first thickness is sandwiched between two current collectors. This assembly is further sandwiched between two layers of silver vanadium oxide, binder and conductive diluent, in the form of a second structure of a second thickness, different than the first thickness. It is known that SVO can be pressed into cohesive structures that readily adhere to a current collector without the presence of binder and conductive diluents. As a result, lithium cells with cathodes of this configuration have the same discharge rate capability as that of conventional Li/SVO cells. At the same time, when the intermediate first structure is thicker than the second structures and/or devoid of non-active materials, these cells exhibit higher capacities than that of conventional Li/SVO cells due to the increased amount of active material in the first structure.
Accordingly, one object of the present invention is to improve the performance of lithium electrochemical cells by providing a new concept in electrode design. Further objects of this invention include providing a cell design for improving the capacity and utilization efficiency of lithium-containing cells.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.